Electrolytic cell

ABSTRACT

An improved electrolytic cell having collector elements arranged horizontally in the cell and extending to the sides thereof, and a slab of aluminum disposed horizontally at the bottom of the cell to which the collector elements are connected.

[ Feb. 8, 1972 United States Patent Johnson [541 ELECTROLYTIC CELL3,063,919 11/1962 Jouguetetal..................... 3,385,778 5/1968Johnson............................

Primary Examiner-John H. Mack Assistant Examiner-D. R. Valentine [22]Filed:

Attorney-Pennie, Edmonds, Morton, Taylor and Adams [57] ABSTRACT Animproved electrolytic cell having collector elements ar- .204/67,204/243 M, 204/244 ------C 3/ (322d ranged horizontally in the cell andextending to the sides Field of 243-247, th r of, and a slab of aluminumdisposed horizontally at the 2 4/2 M bottom of the cell to which thecollector elements are connected.

[51] Int.

29 Claims, 6 Drawing Figures Reierences Cited UNITED STATES PATENTS2,999,801 9/1961 Wleugel................................

ELECTROLYTIC CELL BACKGROUND OFTI-IE INVENTION In. the electrolyticreduction of fusions to produce aluminum in electrolytic cells disposedin a potline, the concept of carrying current from the collectorelements through the steel potshell sides and to the pot-to-pot bus wasincorporated in the'earliest Hall-cells. In'these early constructions,the collector elements .were bolted to the potshell sides which were, inturn, riveted to a bottom much thinner than the sides. This resulted inpractically all the current flowing horizontally along the potshellsides in a manner causing high vertical magnetic flux density in thecavity containing the fused electrolyte and underlyingreduced-aluminum.This high flux density was especially prevalent at the so-called longarm or upstream corners of the cell where current was collected in acopper bus leading endwise of the cell to join the short armbus,'connected to the downstream-comers of-the cell. The flow of currentaround" the sides of cells of this construction, being generally at thelevel-of the'cavity containing the molten aluminum, produces unwantedheaping of the molten metal-and unpredictable electromagneticcirculation of the molten metal andotherwise decreasesthe efficiency ofthe reducing operation.

Many of todays potlines have vertical magnetic flux densities-in themoltenmetal'pad equal to or greater than those of back and forth onthemselves but such methods require much more prefabricated aluminum buswhich is expensive to fabricate and requires considerable space.

Illustrations of cells having lateral cathode and anode bus structureand utilizing either Soderberg or prebaked electrodes in end-to-end orside-by-side arrangement in the potlines are containedin'thefollowingU.S. Pat. Nos.: 2,761,830; 2,804,429; 2,874,ll; 3,385,778; 3,404,081 and3,415,724. In addition to the constructions disclosed in theabove-listed patents, my earlier US. Pat. Nos. 3,434,957 and 3,434,958disclose cell constructions with improved flux density patterns producedby directing the current directly downwardly throughthe potlining to anunderlying slab of aluminum. Constructions of this type, however,require vertically disposed collector elements and thus completeoverhauling of. existing cells using.horizontally disposed collectorelements.

SUMMARY OF THE INVENTION The improved'electrolytic cell constructed inaccordance with. the teachings of the present invention may beadvantageously used in the construction of new potlines of high amperagesuch as 200,000 amperes or more and may also be used for converting atlow cost potshells now in use.

Generally, the improved electrolytic cell includes a rectangular steelor aluminum potshell having a cathode carbon potlining with a cavityfor'containing fused electrolyte'and an underlying layer of reducedaluminum. Horizontally disposed iron collector elements are embedded inthe potlining and extend normal to the sides of the cell for collectingcathode current'and leading it to the pot-to-pot bus means-connectingthe cell to the next cell in a line of cells. Instead of collecting thecurrent flowing from the collector elements in-the conventional cathodering bus encircling the cell at the level ofthe cavity, this structureis eliminated and the current is all directed from the collectorelements to a slab of aluminum disposed at the bottom of the cell inunderlying relationship with the cavity. Portions of the slabextendalong certain areas.

of the sides of the cell for connection to the collector elements andthe slab is shapedso as to direct current'from the collector elementsalong paths which produce a vertical electromagnetic flux density asmeasured along the potlining cavity which is generally uniform along thecavity and less than the horizontal electromagnetic flux density.

The improved electrolytic cell construction not only eliminates the needfor complicated-cathode ring bus structurebut also eliminates the needfor the usual flexibles which are normally used to attach the collectorelements to the bus structure. The elimination of the horizontallyrunning bus structure generally reduces unwanted electromagneticcirculation and heaping of the molten metal in the cavity and providesmore room between potshells for repairs and maintenance. Also, with thecurrent flow being. controlled to produce low vertical electromagneticfluxdensity, the reduction process is made more efficient due to theless heaping of the molten aluminum. The anodes require little if anyadjustment due to uneven burning in different parts of the cell.Theoretically, with a level molten cathode, each anode should adjustitself to draw current at equal current densities which is not possiblewith conventionalconstructions due to the unpredictable heaping of themolten aluminum. The efficiency of the reduction process isalsoincreased since the temperature under the anodes can-be maintainedmore uniform due to the reduction in the magnitude of the vertical fluxdensity and the creation of more uniform flux density throughout thecavity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph comparing thevertical flux density produced in the improved cell of the presentinvention with the flux density of conventional cells;

FIG. 2 is a cross-sectional view of one embodiment of the improvedcell'ofthe present invention;

FIG. 3 is a plan view showing the orientation of the improved cell ofFIG. 2 disposed in side-by-side relation in a line of cells;

FIG. 4 is a plan view of the aluminum slab employed in the embodiment ofthe present invention shown in FIG. 2 with the side portions thereoffolded outwardly and showing the amount and direction of currentflowtherethrough;

FIG. 5 is a plan view of a modified embodiment of the aluminum slabstructure used in the improved cell of the present invention; and

FIG. 6 is a perspective view of the improved collector element of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shownin FIGS. 1 and 2, theimproved electrolytic cell as constructed for side-by-side arrangementin a potline includes a generally rectangular potshell 1 havingrelatively upright sides defined by elongated. sidewalls 2 and 3connected together by end walls 4 and 5 and a generally flat bottom 6.The-potshell contains an electrically conductive cathode potlining 7having a cavity 8 for holding fused electrolyte 9 and an underlyinglayerof reduced molten aluminum 10. The fused electrolyte has a crust 9'frozen on its upper surface. An inwardly extending rim on the potshellforms a reinforced deck plate 11 which is bolted down after thecarbonaceous potlining is rammed into place. Disposed at the bottom ofthe cell and in underlying relation with the cavity 8 is a slab ofaluminum 12. The aluminum slab includes portions extending along certain'areas at. the side of the cell along the sidewalls and end walls aswill be more fully described below. Disposed between the slab ofaluminum and the potlining is a layer of heat insulation brick 13. Thereis also a thin refractory electric insulation 14 under the aluminum slabto prevent the slab from contributing to the electrical conductivity ofthe underlining bottomzof the potshell. The insulation 14 is, however,thin enough to permit the flow of heat from the slab to the bottom ofthe potshell.

Extending through the crust-9' and into the fused electrolytecontained-in the cavity 8 are pairs of carbon anodes 15. Multiplepairs'of anodes extend the length of the cell and are disposed at adistance of about 1.5 inches above the molten aluminum. Asshown in FIG.1, the anodes are mechanically 8 Average having an encircling busstructure at the level of the cavity is 19 made to show the points atwhich the vertical flux density is where a multiple number of flexiblealuminum sheets 20 are welded for-lowing the bus bar to a riser bus 21connecting Table I below sets forth the values of the vertical fluxdensity at eight points along the upstream edge and downstream edge ofthe conventional cell arranged in side-by-side relation in a potline.The values for the eight points along the upstream edge and downstreamedges represent eight points disposed on both sides of the transversecenterline of the cell. The flux alue as the density on the left half ofthe cavity and is only op ppsite in sign or From the values of Table I,the net vertical flux density along the upstream and downstream edges ofthe cell is In FIG. 1, curves representing the vertical flux densityalong the upstream and downstream edges of the improved cell con- ITable II below sets forth the values of the vertical flux density onwhich curves 26 and 27 are based. The values were calculated at the sameeight points in the cavity of the cell as patlining cavity bus structureconnecting the cell in line with other cells. Preparatory to determiningthe shape of the slab shown in FIG. 4, evaluation of the flux density ofthe conventional cell de most troublesome.

density at any point on the right half of the cavity is the same 15 in vplotted in FIG. I. The values of the vertical flux density of the shownby the dotted lines 26 while the values of the vertical flux densityalong the downstream edge of the conventional cell are shown by thedotted lines 27. In FIG. 4, the outline of the potshell is shown by thedash lines 24 and the points along the edges of the cavity 8 where thevertical flux density is mea-.

.Sured r h wn xt eee ir lei nu struction of the present invention areshown by the solid lines 28 and 29, respectively. The curves 28 and 29are calculated in the same manner as used for the curves 26 and 27.

were used for determining the flux density in the conventional cell.

Vertical flux density components in gauss calculated at points alongleft upstream edge of In accordance with the teachings of the presentinvention, the current is withdrawn from the cell through a plurality ofhorizontal collector elements 23 disposed rigidly normal to the sides ofthe cell as by welding their ends to the cell sides.

FIG. 4 shows the particular construction of the aluminum slab used inthe embodiment of the invention shown in FIG. 1.

The particular shape of the aluminum slab including the portions to bedisposed below the cell and those to be disposed along its sides,produces a substantially uniform and low elec- Conductor causingvertical flux density joined to stubs 16. The stubs which may be made ofsteel are, in turn, connected as by welding or bolting to one endof rodmembers 17. The rod members which may be of aluminum or copper areadjustably secured at their other ends to an ano bus bar 18. The bus isformed into a ring bus at its ends cell to the downstream bus 22 of thenext upstream cell in the potline. A superstructure from which the anodering bus is suspended by vertically adjustable jacks is not shown. Asshown in FIG. I, the downstream bus 22 is connected to the cell aluminumslab underlying the cell. Referring to FIG. 3, it will be seen that eachcell includes two downstream bus means 22 extending from the downstreamside of the cell in diverging relation with respect to each other.

downwardly into the underlying portion of the aluminum slab l2 and fromthere to the downstream bus means 22.

In FIG. 4, the aluminum slab is shown in plan view with the sideportions which would extend along the sides of the cell foldedoutwardly. In FIG. 4, the outline of the potshell is shown by the dashline 24. Also, the outline of the edges of the cavity 8 is shown by thedash line 25.

tromagnetic flux density along the cavity. The shape is determined on atrial and error basis by calculating the vertical flux density atvarious points in the cavity as caused by current flowing throughdifferent segments of thecell and adjoining TABLE I.-VE RTICAL MAGNETICFLUX DENSITY OF CONVENTIONAL CELL IN SID E-BY-SIDE ARRANGEMENT INPOTLINE Current in conductor, (M amps.) 1

34 56 055 4nm w2 2 3 L2 wnaumw maaaea Vertical flux density compontentsin gauss calculated at points along left downstream edge of potliningcavity Totalflux downward Totalfluxupward Net vertical flux density.

26706959 200 1 0 &0 &

323 455 42 23 0m0mL2 1 Dim- 2 2 1 in pstream riser to anode bus. End busfrom upstream lateral Pot-to-pot bus segment. Downstream lateral busAdjacent downstream lateral bus Downstream anode bus.. Upstream anodebus.

TABLE IL-VERTICAL MAGNETIC FLUX DENSITY OF IMPROVED CELL OF PRESENTINVENTION IN SIDE-BY-SIDE Vertical flux density components In gausscalculated at points along left upstream edge of potlining cavity 8Average ARRANGEMENT IN POTLINE Total flux downward Total flux upward Netvertical flux density" Conductor causing vertical flux density Currentin 35527241 am mnZsaa VERTICAL MAGNETIC FLUX DENSITY OF IMPROVED CELL OFPRESENT INVENTION IN SIDE-BY-SIDE ARRANGEMENT IN POTLINE-ContinuedVertical flux density components in gauss calculated at points alongleft upstream edge urreul. in f tlin' t conductor, Conductor causingvertical p0 mg ca y (M amps.) flux density 1 2 3 4 5 6 T 5 AverageVertical flux density components in gauss calculated at points alongleft downstream edge of potlim'ng cavity 60.... Upstream riser to anodebus. 6.8 8.3 7.4 6.1 4. 5 3. 5 2. 2 0.6 4.9 02-0-62 Ends of potshell....13. 9 3. 9 1. 5 0. 7 0. 4 0. 2 0.1 0. 0 2. 6 0-12.5-0 Middle bottomslab. 6.1 10.3 13. 7 15.3 13. 6 10. 5 6. 7 1. 1 9. 7 50-.. Pot-to-potbus at so 1.7 8.0 6.9 10.3 9.4 7.2 6.3 2.6 0. 4 .6.2-0-6.2 Downstreampotshell side. 0.-& 1.1 3. 6 12. 7 -21. 2 19.4 -10. 4 2. 7 8. 9 -0Downstream anode bus... 4.3 4.6 4.6 4.0 3.3 2.4 -1. 5 0. 5 3. 1 26-0Upstream anode bus 7.2 7.6 7.4 -6.7 -l5.5 4.1 2. 5 0.9 -6.2

Total flux downward 43. 7 21.3 16.4 23.4 30. 0 25. 9 -14.4 4. 1 22. 3Total flux upward 26. 8 22. 5 29. 5 32. 4 27. 9 21. 4 15. 3 4. 8 22. 6

Net vertical flux density 16. 9 1. 2 9. 0 4. 5 0. 9 0.2 0.2

In determining the vertical flux densities of both the conventional andthe improved cell, a cell having a length of 25% feet and a width of 8feet has been used. Also, 100,000 amperes is the potline electricalcurrent on which Tables I and II and FIG. 1 are based. The calculationnecessary to determine the values for the vertical magnetic flux densityarising from current flowing through different parts of the cell arewell known, and therefore, not described in detail herein. In makingthese calculations, however, one skilled in the art will recognize thatthe vertical flux density component of flux at any certain point alongone of the edges of the cavity 8; namely, at any encircled point (l-8),is proportional to the currentcarrying segment of the conductorstructure under consideration and to the differences in the values ofthe sines of the angles formed, first, between a perpendicular lineextending from the point to the segment under consideration, or to thesegment extended, and a line drawn from the point to the far end of thesegment under consideration; and, secondly, the sine of the angle formedbetween the above-mentioned perpendicular line and a line drawn from thepoint to the near end of the current conductor segment underconsideration. Similarly, the vertical flux density caused is directlyproportional to the ratio h/a where:

a is the perpendicular distance from the point at which the verticalflux density is to be determined to the segment under consideration, andh is the length of the horizontal side of a right triangle constructedin a vertical plane through line a with line u having the hypotenuse ofthe triangle. When a current conductor is at the same level as the pointalong the cavity edge where the flux density is being measured, h=a andthe vertical flux density at this point is at a maximum as compared tothe horizontal flux density which is zero. Also, the vertical fluxdensity at this point is directly proportional to the current in thesegment under consideration and to the above calculated sine differencesand is inversely proportional to the perpendicular distance a from thepoint to the segment.

When a current-carrying segment is directly beneath and parallel to theline of encircled points (1-8) along the edge of the cavity, h=0.Accordingly, the vertical flux density is zero at'these pointsregardless of the amount of current carried in the conductor segment.

When a comparatively long conductor extends on bothsides and isrelatively close to a particular point in the cavity at which the fluxdensity is being measured, the sine differences between the angles ascalculated in the manner described above approaches 2.0 which is the sumof the sines of two 90 angles and the maximum effect which can beexerted by a straight conductor with a given current. The smaller theangle subtended between the lines drawn from the point at which the fluxdensity is to be measured to the opposite ends of the current-carryingsegment of the conductor under consideration, the less the sinedifferences defined above andthe less the effect of the current in thesegment on the vertical flux density at the point under consideration.

The aluminum slab shown in FIG. 4 is disposed directly underneath thepotshell cavity with the side portions disposed along the sidewalls andend walls of the potshell. This construction compels the current to flowin the amounts and directions indicated in FIG. 4. The current values inFIG. 4 are in thousands of amperes; and for purposes of simplicity noelectrical current is shown flowing in the steel potshell where it isbare of aluminum since the amount of such current is small compared tothat current in the aluminum slab. For example, in practice it may beassumed that steel has an electrical conductivity of about 20 percent ofthat of aluminum. Accordingly, using an aluminum slab which is thickenough to result in a current density of 400 amperes per square inch ofaluminum conductor cross section, the steel potshell can be assumed tobe conducting amperes per square inch of aluminum. With a potshellbottom of five-eighths of an inch thick steel and an aluminum slab caston top of the bottom and having a thickness of 1% inches, the latterwill carry percent of the current.

Also, in connection with determining the flux density values of TableII, the distortion of the magnetic flux pattern at the various points ofmeasurement as caused by the magnetic permeability of the steel potshellis not reflected. The magnetic flux densities as calculated in Table IIare nevertheless accurate indications of the relative values of thedensities which can be expected to be measured in an actual operatingcell. In this regard, it is to be noted that in accordance with theteachings of the present invention, aluminum may be used for all partsof the potshell in place of the steel so as to reduce the distortion ofthe flux pattern which would otherwise be caused by steel.

With particular reference to Table I above, an examination of thevertical flux densities caused by each current-carrying segmentconsidered in the conventional cell shows that the greatest problem inneutralizing flux density exists along the upstream edge of the cellcavity. Here, on the left end of the cell when facing upstream, thedownstream lateral bus structure contributes a negative (downwarddirection) flux of 10.0 gauss compared to the 204.9 gauss in a positive(upward direction) contributed by the seven other conductor segmentsconsidered in the estimates. The flux density variation as produced atthis point, as well as the other points along the upstream anddownstream edges of the cavity is reduced by eliminating theconventional external bus structure and replacing it with the aluminumslab structure shown in FIG. 4.

The aluminum slab includes a solid middle bottom portion 30 extendingthe full length of the potshell and a solid downstream bottom portion 31also extending the full length of the potshell. Along the sides of thecell, the aluminum slab includes solid comer portions 32 and fingerportions 33. The portions of the slab disposed on one side of thetransverse centerline of the potshell adjacent one end wall areseparated electrically from the portions of the slab on the other sideof such centerline. The collector elements 23 extending to the sidewallsand end walls of the potshell are connected directly to the alignedportions of the aluminum slab detailed in FIG. 4.

As shown in FIG. 3, there are fourteen collector elements extending toeach of the sidewalls of the potshell while there are four collectorelements extending to each of the end walls of the potshelluAs shown byTable II, the middle bottom slab portion-30 and the downstream bottomslab portion 31 both contribute downward vertical flux densities tobalance the excess upward flux density at the various points along theupstream edge of the cavity. Additional balancing is achieved by thefour collector elements extending inwardly from each end wall of thepotshell with the electrical current from these being routed along thesidewalls of the potshell.

The currents flowing along the four potshell corner portions 32 of theslab are at the level of the cavity and hence exert the maximum fluxbalancing effect thereon at points near the ends of the cavity. Also,the aluminum finger portions of the slab are electrically insulated fromthe potshell with a dielectric paint or thin coating and thus contributenothing to longitudinalconduction of current through the potshellsidewalls.

To compel the side portions of the aluminum slab disposed along thedownstream side of the cell to carry the current in a circuit which iselectrically parallel to the bottom slab portion, the thickness of theside and bottom portions are correlated so that the current densities inboth will be about equal. With the slab having a downstream side portionof 3 feet in height and carrying 4,690 amperes and a bottom portiontotaling feet in width and carrying 14,062 amperes, the side portion ofthe slab will be 1% inches thick while the bottom portion of the slabwill be 2% inches thick. With this construction, both the downstreamside portion and bottom portion of the slab will carry I04 amperes persquare inch. By proportioning the thickness in this way for a currenttravel in parallel paths, each segment is compelled to carry the currentdesired so as to achieve a balance of the flux density.

The amount and direction of vertical flux density at the various pointsof the cavity is also dependent on the construction and arrangement ofthe external bus structure connecting the adjacent cells in the potlinetogether. With the construction shown in FIGS. 2, 3 and 4, thedownstream bus means 22 includes two arms 22a and 2211 each of which isconnected to one of the separate bottom portions of the aluminum slabstructure disposed on either side of the centerline of the cell. Theconnection of the arms is made to the bottom portions of the slab atpoints spaced from the adjacent end walls and the arms extend toward thenext downstream cell in a diverging relation to each other as mostclearly shown in FIG. 3. The arms extend at a level below the cell untilreaching the upstream side of the next downstream cell at which pointthe arms are connected to the riser bus means 21.

By comparing the dotted and solid curves of FIG. 1 and the values setout in Tables I and II, it is seen that with the cell construction ofthe present invention, the average vertical flux density along theupstream edge of the cavity of the cell is reduced by about 90 percent(from 95.8 to 9.0 gauss). On the downstream cavity edge, the reductionin the average vertical flux density with the improved cell of thepresent invention is from -24 gauss to 0.2 gauss.

With the improved cell construction of FIGS. 2-4 of the presentinvention, the usual insulation of the collector elements from thepotshell sides as found in conventional cells is eliminated. Instead,collector elements 23 are firmly welded to the potshell to make anelectrical connection. The collector elements are also welded to thealuminum slab portions with which they are aligned. In the preferredconstruction, the collector elements are wedge-shaped as shown in FIG.6. Where the potlining is built up of prebaked carbon blocks, theseblocks are made with a wedge-shaped slot. Then, collector elementshaving a uniform rectangular cross section are positioned within theseslots and anchored by pouring in cast iron. This produces a wroughtiron-cast iron bar with the desired wedge-shaped horizontal crosssection. The wedge-shaped collector elements when firmly welded to theside portions of the aluminum slab and to the sidewalls and end walls ofthe potshelltend to make a tighter connection, thus, one with lowerelectrical loses, as the bar lengthens when heating from roomtemperature to l,000 C. and as the potlining expands during operationand pushes outwardly against the pot sides. The wedge-shapedconstruction prevents the potlining from shearing away from thecollector elements as would tend to occur with conventional collectorelements of uniform cross section.

In addition to the connection of the collector elements to the walls ofthe potcell as well as to the aluminum slab, all or part of theinsulation normally disposed between the carbonaceous potlining and thesteel potshell of conventional cells may be omitted to permit current toflow laterally to the sidewalls of the potshell.

Removal of the insulation is most advantageously effected where the wallis aligned with regions in the potlining which are in turn adjacent theportions of the cavity where the vertical electromagnetic flux densityis low in relation to the density in the other areas of the cavity. Withconventional cell constructions, lateral conduction of current hasgenerally been avoided since with high vertical flux density in thecavity, lateral current produces the undesired electromagneticcirculation and heaping of the molten metal in unpredictable patterns.With the improved cell construction where low vertical flux densitiesare attained, it is no longer objectionable to conduct some of thecurrent laterally through the potlining and molten metal contained inthe cavity. In order to ensure a good electrical contact of the sideportions of the slab with the carbon potlining, such portions are formedwith a corrugated inner surface when cast onto the potshell sides.

Where the potshell sidewalls and end walls are not covered by sideportions of the aluminum slab, the carbonaceous potlining will be rammedtightly against the potshell walls so that part of the electric currentwill readily flow thereto. Also, the collector elements of the improvedcell construction are made shorter than the conventional collectorelements with the advantages that the inner ends of the collectorelements are not disposed along the longitudinal centerline of thepotshell. I-Iere heaving of the potlining and molten aluminumpenetrating through the resulting cracks and dissolving collectorelements is usually most severe.

The shape of the aluminum slabs shown in FIG. 4 has been devised toreduce the variations in the vertical flux densities at different pointsalong the cavity of the potshell as well as for the purpose ofminimizing these densities. By orienting the conducting segments of thecell as shown in FIG. 4, the vertical flux density at the upstream anddownstream edges of the cavity is made substantially uniform as shown bythe solid curves in FIG. 1. Also, the value of these densities is keptgenerally less than 30 gauss and the average under 10 gauss.

The shape of the aluminum slab and associated bus structure has, bychanneling the current flow into paths that produce low vertical fluxdensity, increased the horizontal flux density in the improved cell ascompared to that which would be found in conventional cells. Thehorizontal flux density does not, however, produce the undesired heapingand electromagnetic circulation as caused by the vertical flux densityand, therefore, is not objectionable.

The shape of the aluminum slab shown in FIG. 4 produces the desiredresult of reducing and evening out the electromagnetic vertical fluxdensities in the cell. However, it is to be understood that othersimilar shapes may be used as long as they produce similarly effectivecurrent flow patterns.

In FIG. 5 there is shown an aluminum slab construction 34 adapted forminimizing and evening out the magnetic vertical flux densities in cellsadapted to be disposed in end-to-end arrangement in a potline. The shapeof the slab is determined in the same way as the shape of the slab shownin FIG. 4 is calculated. Table'lll below shows the calculations of thevertical flux densities caused by current'flowing through variousconductor segments of the cell and adjacent bus structure.

b. said slab is shaped to direct the current from said collectorelements and through said slab to produce a substantially uniformvertical electromagnetic flux density as TABLE ill. --INVENTION IO'ICELLMAGNETIC FLUX DENSITY IN END-'IO-END ARRANGEMENT IN POTLINE Verticalflux donsltycomponents in gauss calculated at'polnts along right edge ofpotlinlng cavity when one laces upstream and Conductor causing verticalflux density segment 1 2 3 4 6 6 7 8 9 10 l1 l2 l3 14 Avg Near side slab24.0 -31.0 36. 5 --40.6 42.0 -42.6 42.8 42.9 -42.9 -42.8 42. 6 4l.9 40.335.4 39.2 Near bottom slab. 0 0 0 0 0 0 0 0 O 0 0 0 0 0 Near midbottomslab 0.1 0.1 0.1 0.1 0. 2 0. 2 0.5 0. 9 1.8 3. 7 6. 7 0.7 12. 2 12.3 a.Far mldbottom slabm- 0. 1 0. 1 0. 2 0. 2 0. 3 0. 5 0. 7 1. 2 2. 0 3. 35. 0 6.7 7. 7 7. 4 2. 5 For bottom slab-.. 1.3 1.9 2.7 3.7 5.1 6.0 8.19.2 10.7 11.4 11.7 11.4 10.6 0.2 7.4 Far side slab 4. 6 5. 5 6. 4 7.0 7.5 7. 8 8.0 3. 1 8.0 7. 9 7. 6 7. 2 6. 5 5. 6 7. 0 End side or bottomslab" -8.3 3.3 l.2 0.5 0.3 0 0 0 0 0. 5 -l.3 -3.0 -6.7 7.8 '2.3Collector bars. 3.3 0 3.3 0 0 0 0 O 0 0 0 O 0 0 0 Near anode bus 7. 5 8.6 8. 7 8. 4 7. 8 7. l 6, 4 5. 5 4. 7 3. 7 3.0 2. 6 1.6 1.0 6. 5 Faranode bus-- 19.0 21.4 22.4 22.3 21.4 19.0 18.0 16.0 13.8 11.6 9. 5 7. 55.7 4.2 15.1

Total flux down. -'-32.3 34.3 -41.0 4l. 1 42.3 -42.6 42.8 42.9 -42.943.3 43.8 -44.9 --47.0 43.2 4l.5 Total flux up 35.9 37.5 40. 5 41.7 42.341.2 41.7 40.9 41.0 41.6 43. 5 45.1 44.3 39.7 41.0

Net vertical flux density in gauss 3.6 3.2 0.5 0.5 0 -1.4 -1. 1 -2.0 1.9-1.7 -0.3 0.2 -2.7 -3.5 -0.5

As shown in FIG. 5, the slab when used in a cell adapted to measuredalong the potlining cavity. be disposed end-to-end relation in a potlinehas a bottom por- 4. The improvement in an electrolytic cell accordingto tion 35 extending inwardly of the opposite sidewalls of the cellclaim 3 wherein: by progressively smaller distances as measured alongpoints a. said slab is shaped to direct the current from saidcolleclocated further from the downstream end of the cell. Also, the torelements through said slab to produce a vertical elecdownstream end ofthe cell, the bottom portion 35 of the alutromagnetic flux density asmeasured along the potlining minum slab extends completely across thebottom of the cavity which is generally less than the horizontalelecpotshell. The bottom portion of the slab terminates at a tromagneticflux density therealong. distance spaced from the other end wall of thepotshell so that 5. The improvement in an electrolytic cell according tothere is no underlying portion of the slab disposed along the claim 4wherein: upstream end of the cell. Also, the portions 36 and 37 of thea. the slab is shaped to produce a vertical electromagnetic slabdisposed along the sides of the cell extend the full length 5 fluxdensity as caused by external current flow through of the sidewalls witha diminishing height adjacent the upadjacent cells and connecting busmeans when disposed stream side of the cell. in said line which issubstantially uniform and generally The slab construction shown in FIG.5 is-completed by the less than the horizontal electromagnetic fluxdensity as upstanding end portions disposed along either end wall of thecaused by said external current.

'potshell. More particularly, the slab includes an upstandi 40 6. Theimprovement in an electrolytic cell according to portion 38 disposedcentrally of the end wall forming the Claim 5 Whefem! downstream end ofthe cell and two upstanding po i 39' a. said vertical magnetic fluxdensity as measured at difalong the upstream end wall of the cell. Asshown, the portions ferent points along the potlining cavity averages no39 are disposed adjacent the comers of the cell which are gr ter th nabout lOgauss.

formed with the sidewalls. As with the embodiment of the in- Thimprovement in an electrolytic cell according to ventionshown in FIGS.2-4, the collector elements 40 are claimSwherein:

connected directly to the aluminum slab structure aligned a. saidpotshell is generally rectangular with elongated h i h sidewallsconnected together by end walls, said potshell Iclaim: being adapted tobe disposed in side-by-side relation with l. in an electrolytic cell forthe reduction of aluminum ad-lacent potshenS m sadlfne?and adapted'to bedisposed-in aline of electrolytic cells in electrithe pomms 9 531d slabdlsposed to P Side of the cally connected relationshipand having apotshell with relaverse cemeflme of the Potshe *f f end wall tivelyupright sides and a generally flat bottom, a slab of aluthereof: are fthe P of Said Slab the minum at said bottom, a layer-of refractorythermal insulation other s1de of sad cellteflma over the Slab, anelectrically conductive cathode potlining The improvement in anelectrolytic cell according to having a cavity for containingfusedelectrolyte and underlying clam 5 wherem1 reducedaluminum, a pluralityof cathode collector elements f d P -Y Tecmngular with elongatedextending generally normal to at least some of said sides and in Fconnected PE K Y walls, Sald Potshell :electrical engagement with saidpotlining, and cathode bus befng adapted to f P l'- relation i h meansfor connecting said cell to the anode structure-of the adlacem Potshells531d 11116; and

next downstream cell in said line, the improvement wherein: Said lincludes:

a. said collector elements at said sides of the cell are electrimidfflebottom Portions disposed on pp cally connected to said slab forconnection through said sides of said transverse centerline and alongthe middle slab to the anode structure of the next downstream cell. of Wbottom of the Potshell,

2. The improvement in an electrolytic cell according to solld downstwambottom Portions disposed P- claim wherein: posite sides of saidtransverse centerline and along the a. said cathode bus means includestwo arms each of which Side Of i potshell adapted to be the down treamid is connected to the slab on opposite sides of the trans- Ofthe cellin Said line w verse centerline of the cell and both of which extendCome! p i disposed along the sidewalls and end toward the downstreamside of the cell in diverging rela- W l at h rn r f Said potshell andelectrically tion to each other. connected to the collector elementsextending to the 3. The improvement in an electrolytic cell according toside and end walls adjacent said corners, and

l i 1 wh i .4. finger portions electrically connecting the remainder ofa. said slab extends along certain areas of the sides of said saidcollector elements to said middle and downstream i al..-

bottom portions.

9. The improvement in an electrolytic cell according to claim 8 wherein:

a. the portions of said slab disposed to one side of the transversecenterline of the potshell adjacent one end wall thereof are separatefrom the portions of said slab on the other sideof said centerline.

10. The improvement in an electrolytic cell according to claim 9wherein:

a. the finger portions connected to the collector elements at thesidewalls of the potshell are contained in planes disposed perpendicularto said sidewalls.

ll. The improvement in an electrolytic cell according to claim 10wherein:

a. said collector elements are electrically connected to said potshell.

12. The improvement in an electrolytic cell according to claim 11wherein:

a. said slab is electrically insulated from said potshell.

13. The improvement in an electrolytic cell according to claim 3wherein:

a. said potshell is generally rectangular with elongated sidewallsconnected together by end walls, said potshell being adapted to bedisposed in side-by-side relation with adjacent potshells in said line;and

b. said slab includes a corner end portion disposed along the end wallof said potshell and a comer side portion disposed along the sidewall ofsaid potshell and in electrical contact with said comer end portion.

14. The improvement in an electrolytic cell according to' claim 13wherein:

a. each of said comer end portions is connected to the bottom portionsof said slab solely through its connection with the associated comerside portion of said slab.

15. The improvement in an electrolytic cell according to claim 14wherein: v

a. the corner side portions of said slab along the sidewall thereof,which is adapted to define the downstream side of the cell insaid line,extend further toward the transverse centerline of said potshell thanthe comer side portions along the opposite sidewall.

16. The improvement in an electrolytic cell according to claim 3wherein:

a. said slab is disposed along the inner surface of said potshell; and

b. the side portions of said slab have a corrugated surface defining theinterface with the potlining with the corrugations extending in thedirection of current flow therethrough.

17. The improvement in an electrolytic cell according to claim 3wherein:

a. said potshell is in electrical contact with said potlining along theinner side and end walls thereof.

18. The improvement in an electrolytic cell according to claim 3wherein:

a. said collector elements are tapered in horizontal cross section in adirection away from the sides of the cell and are mechanically andelectrically attached to the sides of said cell.

19. The improvement in an electrolytic cell according to being adaptedto be disposed in end-to-end relation with 70 adjacent potshells in saidline; and

b. the portion of said slab at the bottom of said potshell extendsinwardly from the opposite sidewalls by progressively smaller distancesat points located further from the one end wall adapted to define thedownstream side of the cell when disposed in said line of cells.

21. The improvement in an electrolytic cell according to claim 3wherein:

a. said potshell is generally rectangular with elongated sidewallsconnected together by end walls, said potshell being adapted to bedisposed in end-to-end relation with adjacent potshells in said line;and

b. the portion of the slab at the bottom of the potshell:

1. extends completely across said bottom adjacent the one end walladapted to define the downstream side of the cell,

2. extends inwardly from the opposite sidewalls by progressively smallerdistances at points located further from said one end wall; and

3. terminates at a point spaced from the other end wall of saidpotshell.

22. The improvement in an electrolytic cell according to claim 21wherein:

a. the portions of the slab disposed along the sidewalls of the potshellextend the full length of the sidewalls with diminishing height adjacentthe other end wall of the potshell; and

b. the collector elements extending normal to the sidewalls areconnected to the aligned side portions of the slab.

23. The improvement in an electrolytic cell according to claim 22wherein: Y

a. the side portions of the slab further extend centrally of the one endwall and along the other end wall adjacent the corners formed with thesidewalls.

24. in the production of aluminum by the electrolytic reduction offluoride fusions of alumina contained in a cavity in a rectangular celladapted to be disposed in a line of cells and having upright metal sidesand a flat bottom and lined at least in part with an electricallyconductive lining which serves as a cathode container for theelectrolyte and molten aluminum during said reduction and from whichcurrent is drawn by horizontally disposed collecting elements to busmeans disposed exteriorly of said cell, the improvement of:

a. directing current from said collecting elements to the sides of saidcell and thereafter downwardly into an aluminum slab disposedhorizontally at the bottom of the cell beneath the fluoride fusion ofalumina and from there to said bus means adapted to lead from underneathsaid cell to the next downstream cell in said line.

25. The method according to claim 24 including:

a. directing said current along paths from said collecting elements tosaid bus means which produce a vertical electromagnetic flux density ascaused by current flowing through said cell and as measured along thecavity which is substantially uniform and generally less than thehorizontal electromagnetic flux density therealong.

26. The method according to claim 25 including:

a. directing said current along paths from said collecting means to saidbus means which produce a vertical electromagnetic flux density ascaused by external current flow through adjacent cells and connectingbus means when disposed in said line which is substantially uniform andgenerally less than the horizontal electromagnetic flux density ascaused by said external current.

27. In the production of aluminum by the electrolytic reduction offluoride fusions of alumina contained in a cavity in a rectangular celladapted to be disposed in a line of cells and having upright metal sidesand a flat bottom and lined at least in part with an electricallyconductive lining which serves as a cathode container for theelectrolyte and molten aluminum during said reduction and from whichcurrent is drawn by horizontally disposed collecting elements to busmeans disposed exteriorly of said cell, the improvement of:

a. directing current laterally from said cavity and to said metalsidewalls through the regions of the potlining adjacent the portions ofthe cavity where the vertical electromagnetic flux density is low inrelation to the density in other portions of the cavity and thereafterto said exteriorly disposed bus means.

lecting elements in the regions of the potlining adjacent said otherportions of the cavity where the vertical electromagnetic flux densityis high and thereafter to said exteriorly disposed bus means.

2. solid downstream bottom portions disposed on opposite sides of saidtransverse centerline and along the side of said potshell adapted to bethe downstream side of the cell in said line of cells,
 2. extendsinwardly from the opposite sidewalls by progressively smaller distancesat points located further from said one end wall; and
 2. The improvementin an electrolytic cell according to claim 1 wherein: a. said cathodebus means includes two arms each of which is connected to the slab onopposite sides of the transverse centerline of the cell and both ofwhich extend toward the downstream side of the cell in divergingrelation to each other.
 3. The improvement in an electrolytic cellaccording to claim 1 wherein: a. said slab extends along certain areasof the sides of said cell; and b. said slab is shaped to direct thecurrent from said collector elements and through said slab to produce asubstantially uniform vertical electromagnetic flux density as measuredalong the potlining cavity.
 3. terminates at a point spaced from theother end wall of said potshell.
 3. corner portions disposed along thesidewalls and end walls at each corner of said potshell and electricallyconnected to the collector elements extending to the side and end wallsadjacent said corners, and
 4. finger portions electrically connectingthe remainder of said collector elements to said middle and downstreambottom portions.
 4. The improvement in an electrolytic cell according toclaim 3 wherein: a. said slab is shaped to direct the current from saidcollector elements through said slab to produce a verticalelectromagnetic flux density as measured along the potlining cavitywhich is generally less than the horizontal electromagnetic flux densitytherealong.
 5. The improvement in an electrolytic cell acCording toclaim 4 wherein: a. the slab is shaped to produce a verticalelectromagnetic flux density as caused by external current flow throughadjacent cells and connecting bus means when disposed in said line whichis substantially uniform and generally less than the horizontalelectromagnetic flux density as caused by said external current.
 6. Theimprovement in an electrolytic cell according to claim 5 wherein: a.said vertical magnetic flux density as measured at different pointsalong the potlining cavity averages no greater than about 10 gauss. 7.The improvement in an electrolytic cell according to claim 5 wherein: a.said potshell is generally rectangular with elongated sidewallsconnected together by end walls, said potshell being adapted to bedisposed in side-by-side relation with adjacent potshells in said line;and b. the portions of said slab disposed to one side of the transversecenterline of the potshell adjacent one end wall thereof are separatefrom the portions of said slab on the other side of said centerline. 8.The improvement in an electrolytic cell according to claim 5 wherein: a.said potshell is generally rectangular with elongated sidewallsconnected together by end walls, said potshell being adapted to bedisposed in side-by-side relation with adjacent potshells in said line;and b. said slab includes:
 9. The improvement in an electrolytic cellaccording to claim 8 wherein: a. the portions of said slab disposed toone side of the transverse centerline of the potshell adjacent one endwall thereof are separate from the portions of said slab on the otherside of said centerline.
 10. The improvement in an electrolytic cellaccording to claim 9 wherein: a. the finger portions connected to thecollector elements at the sidewalls of the potshell are contained inplanes disposed perpendicular to said sidewalls.
 11. The improvement inan electrolytic cell according to claim 10 wherein: a. said collectorelements are electrically connected to said potshell.
 12. Theimprovement in an electrolytic cell according to claim 11 wherein: a.said slab is electrically insulated from said potshell.
 13. Theimprovement in an electrolytic cell according to claim 3 wherein: a.said potshell is generally rectangular with elongated sidewallsconnected together by end walls, said potshell being adapted to bedisposed in side-by-side relation with adjacent potshells in said line;and b. said slab includes a corner end portion disposed along the endwall of said potshell and a corner side portion disposed along thesidewall of said potshell and in electrical contact with said corner endportion.
 14. The improvement in an electrolytic cell according to claim13 wherein: a. each of said corner end portions is connected to thebottom portions of said slab solely through its connection with theassociated corner side portion of said slab.
 15. The improvement in anelectrolytic cell according to claim 14 wherein: a. the corner sideportions of said slab along the sidewall thereof, which is adapted todefine the downstream side of the cell in said line, extend furthertoward the transverse centerline of said potshell than the corner sideportions along the oppositE sidewall.
 16. The improvement in anelectrolytic cell according to claim 3 wherein: a. said slab is disposedalong the inner surface of said potshell; and b. the side portions ofsaid slab have a corrugated surface defining the interface with thepotlining with the corrugations extending in the direction of currentflow therethrough.
 17. The improvement in an electrolytic cell accordingto claim 3 wherein: a. said potshell is in electrical contact with saidpotlining along the inner side and end walls thereof.
 18. Theimprovement in an electrolytic cell according to claim 3 wherein: a.said collector elements are tapered in horizontal cross section in adirection away from the sides of the cell and are mechanically andelectrically attached to the sides of said cell.
 19. The improvement inan electrolytic cell according to claim 18 wherein: a. said collectorelements include a central elongated core section of uniform crosssection and an outer section of cast iron formed in situ between saidcore section and the wall surface of the openings in said potlining. 20.The improvement in an electrolytic cell according to claim 3 wherein: a.said potshell is generally rectangular with elongated sidewallsconnected together by end walls, said potshell being adapted to bedisposed in end-to-end relation with adjacent potshells in said line;and b. the portion of said slab at the bottom of said potshell extendsinwardly from the opposite sidewalls by progressively smaller distancesat points located further from the one end wall adapted to define thedownstream side of the cell when disposed in said line of cells.
 21. Theimprovement in an electrolytic cell according to claim 3 wherein: a.said potshell is generally rectangular with elongated sidewallsconnected together by end walls, said potshell being adapted to bedisposed in end-to-end relation with adjacent potshells in said line;and b. the portion of the slab at the bottom of the potshell:
 22. Theimprovement in an electrolytic cell according to claim 21 wherein: a.the portions of the slab disposed along the sidewalls of the potshellextend the full length of the sidewalls with diminishing height adjacentthe other end wall of the potshell; and b. the collector elementsextending normal to the sidewalls are connected to the aligned sideportions of the slab.
 23. The improvement in an electrolytic cellaccording to claim 22 wherein: a. the side portions of the slab furtherextend centrally of the one end wall and along the other end walladjacent the corners formed with the sidewalls.
 24. In the production ofaluminum by the electrolytic reduction of fluoride fusions of aluminacontained in a cavity in a rectangular cell adapted to be disposed in aline of cells and having upright metal sides and a flat bottom and linedat least in part with an electrically conductive lining which serves asa cathode container for the electrolyte and molten aluminum during saidreduction and from which current is drawn by horizontally disposedcollecting elements to bus means disposed exteriorly of said cell, theimprovement of: a. directing current from said collecting elements tothe sides of said cell and thereafter downwardly into an aluminum slabdisposed horizontally at the bottom of the cell beneath the fluoridefusion of alumina and from there to said bus means adapted to lead fromunderneath said cell to the next downstream cell in said line.
 25. Themethod according to claim 24 including: a. directing said current alongpaths from said collecting elements to said bus meaNs which produce avertical electromagnetic flux density as caused by current flowingthrough said cell and as measured along the cavity which issubstantially uniform and generally less than the horizontalelectromagnetic flux density therealong.
 26. The method according toclaim 25 including: a. directing said current along paths from saidcollecting means to said bus means which produce a verticalelectromagnetic flux density as caused by external current flow throughadjacent cells and connecting bus means when disposed in said line whichis substantially uniform and generally less than the horizontalelectromagnetic flux density as caused by said external current.
 27. Inthe production of aluminum by the electrolytic reduction of fluoridefusions of alumina contained in a cavity in a rectangular cell adaptedto be disposed in a line of cells and having upright metal sides and aflat bottom and lined at least in part with an electrically conductivelining which serves as a cathode container for the electrolyte andmolten aluminum during said reduction and from which current is drawn byhorizontally disposed collecting elements to bus means disposedexteriorly of said cell, the improvement of: a. directing currentlaterally from said cavity and to said metal sidewalls through theregions of the potlining adjacent the portions of the cavity where thevertical electromagnetic flux density is low in relation to the densityin other portions of the cavity and thereafter to said exteriorlydisposed bus means.
 28. The method according to claim 27 including: a.directing the current flowing through the metal sidewalls along pathsleading underneath said cell and then to said exteriorly disposed busmeans.
 29. The method according to claim 28 including: a. directingcurrent from said cavity and through said collecting elements in theregions of the potlining adjacent said other portions of the cavitywhere the vertical electromagnetic flux density is high and thereafterto said exteriorly disposed bus means.